The present invention relates to an amplifier operating from a plurality of power source systems in which there is no current flow between the power source systems.
An example of an amplifier operating from a plurality of power sources is shown in FIG. 1. In FIG. 1, reference symbols +V.sub.1 and -V.sub.1 designate voltage amplifying stage power sources, and +V.sub.2 and -V.sub.2 power amplifying stage power sources. A bias resistor R.sub.1 is connected between input terminals 1 and 1'. The terminal 1 is connected to the gate of an FET Q.sub.1. The FET Q.sub.1 and an FET Q.sub.2 form a differential amplifier circuit. The sources of the FETs are connected together and are connected to a constant current source I.sub.1. The drain of the FET Q.sub.1 is connected to one terminal of a load resistor R.sub.2 and to the base of a transistor Q.sub.3. The other terminal of the load resistor R.sub.2 and the emitter of the transistor Q.sub.3 are connected to the power source +V.sub.1. The collector of the transistor Q.sub.3 is connected to a constant current source I.sub.2, and also to the base of a transistor Q.sub.4. The transistor Q.sub.4 and a transistor Q.sub.5 are Darlington connected. The emitters of the transistors Q.sub.4 and Q.sub.5 are connected to constant current sources I.sub.3 and I.sub.4, respectively, and the collectors thereof are connected to the power source +V.sub.2. The emitter of the transistor Q.sub.5 is connected through a resistor R.sub.3 to the gate of the FET Q.sub.2. A resistor R.sub.4 is connected between the gate of the FET Q.sub.2 and the terminal 1'. The connecting point of the power sources +V.sub.1 and -V.sub.1 is connected to the input terminal 1' and is grounded at a grounding point G.sub.1. The emitter of the transistor Q.sub.5 is connected to a terminal 2 which is connected to one terminal of a load 3, the other terminal of which is connected to a terminal 2'. The terminal 2' is connected to the connecting point of the power sources +V.sub.2 and -V.sub.2 and is grounded at a grounding point G.sub.2. The grounding points G.sub.1 and G.sub.2 are connected together through a ground bus or chassis. Further in FIG. 1, reference numeral 4 designates a voltage amplifying stage, and 5 a power amplifying stage.
The operation of the amplifier thus arranged will be described.
The circuit is fundamentally a negative feedback amplifier. If the loop gain is sufficiently high, then the closed loop voltage gain can be represented substantially by (R.sub.3 +R.sub.4)/R.sub.4.
Currents flow in the circuit as follows: The base current (signal current) of the transistor Q.sub.4 flows from the power source +V.sub.1 through the transistor Q.sub.3, the base and emitter of the transistor Q.sub.4 and the base and emitter of the transistor Q.sub.5 to the load 3, and it returns from the load 3 through the terminal 2', the connecting point of the power source +V.sub.2 and -V.sub.2 and the grounding points G.sub.2 and G.sub.1 to the connecting point of the power sources +V.sub.1 and -V.sub.1. Similarly, almost all the current flowing in the feedback resistors R.sub.3 and R.sub.4 flows as indicated by a route i.sub.2 in FIG. 1: the power source +V.sub.2 --the transistor Q.sub.5 --the resistor R.sub.3 --the resistor R.sub.4 --the grounding point G.sub.1 --the grounding point G.sub.2.
In the conventional amplifier constructed as described above, if there is some nonzero impedance between the grounding points G.sub.1 and G.sub.2, a potential difference is produced between the grounding points which adversely affects amplification of the signal. In the case of the negative feedback amplifier, the magnitude of the current i.sub.1 is set in such a manner as to correct the nonlinearity of the transistors Q.sub.4 and Q.sub.5, and therefore, the current i.sub.1 itself includes considerable nonlinear components. Accordingly, the potential difference between the grounding points G.sub.1 and G.sub.2 causes a serious problem. Especially in an audio power amplifier, the input voltage is often very low, and accordingly even a small potential difference between the grounding points G.sub.1 and G.sub.2 adversely affects the performance and harms the tone quality.
Instead of the transistor Q.sub.4, an FET Q.sub.4 may be employed as shown in FIG. 2. In this case, usually cascade connection is employed by using a transistor Q.sub.6. However, the circuit thus modified suffers from the same difficulties as the circuit in FIG. 1 because little gate current flows in the FET Q.sub.4 but the same current i.sub.1 flows to the base of the transistor Q.sub.6.